In continuous casting of molten steel, a flow state of molten steel in a mold for receiving molten steel has a great impact steel quality. Thus, it is an important technical matter for a continuous casting operation to control the flow state in connection with a structure of an immersion nozzle having a direct impact on the flow state.
A configuration of an inner bore of the immersion nozzle, particularly, a configuration of a discharge port of the immersion nozzle, has a great impact on a state of a molten steel stream.
Depending on a state of a molten steel stream from the discharge port, a flow state of molten steel in a mold (in-mold molten steel) becomes unstable due to episodic occurrence of turbulences therein, such as reversed flows in various regions in the mold and locally deflected flows which frequently change with time, and resulting fluctuation (“wave”, “heave”, “change in flow direction”) in a molten steel surface, to cause difficulty in allowing inclusions to sufficiently float up around an edge of a slab and in allowing a mold powder to be uniformly transferred onto a surface of the slab, which leads to non-uniform entrapment/incorporation of the mold powder and the inclusions into the slab.
Moreover, there arises another problem, such as difficulty in obtaining a temperature distribution of in-mold molten steel requited for or optimal to formation of a shell during a course of solidification of the molten steel. These exert a negative impact on slab quality and increase a risk of occurrence of a breakout, etc.
As a prerequisite to solving such problems, it is necessary to take measures, such as maximally uniforming a flow velocity, and preventing occurrence of a deflected flow. However, even if a configuration of the discharge port, such as an angle and an area thereof, is simply adjusted, a stable molten steel stream free of mold powder entrapment cannot be obtained
As measures for the above problems, it has been tried to set an angle of a discharge port of an immersion nozzle in an upward direction so as to allow a molten steel stream flowing out of the discharge port of the immersion nozzle to provide a flow in the vicinity of a molten steel surface even at a position adjacent to a periphery of a mold. However, even if an angle of a discharge port formed in a part of a wall of a straight nozzle body is changed within the range of a wall thickness of the straight nozzle body, a sufficiently stable flow cannot be obtained.
For example, as means for controlling a molten steel stream, the following Patent Document 1 proposes an immersion nozzle comprising a discharge port formed in a semicircular shape having a lower region which is a chord equal to an inner diameter of a cylindrical tube, and an upper region which is an arc equal to one-half of an inner circumference of the cylindrical tube. However, even if the discharge port is simply formed in a circular (semicircular) shape or the like in cross-section against a molten-steel outflow direction as in the Patent Document 1, turbulences in a molten steel stream during discharge from the discharge port and non-uniformity in velocity in the cross-section cannot be solved. Thus, the aforementioned various problems, such as mold powder entrapment, still cannot be solved.
The following Patent Document 2 proposes to form a discharge port of an immersion nozzle into a horizontally-long rectangular shape, and set a horizontal-to-vertical ratio of the rectangular shape in the range of 1.01 to 1.20. However, even if the discharge port is simply formed in a rectangular shape in cross-section against a molten-steel outflow direction, or a horizontal-to-vertical ratio of the rectangular shape is simply set in a specific range, turbulences in a molten steel stream during discharge from the discharge port and non-uniformity in velocity in the cross-section cannot be solved. Thus, the aforementioned various problems, such as mold powder entrapment, still cannot be solved.
The following Patent Document 3 discloses a molten steel-introducing submerged entry nozzle for preventing pencil type defects in a casting product, wherein a central bore communicating with an exit port (discharge port) terminates at an upwardly dish-shaped bottom surface which extends to a periphery of a nozzle structure and forms a lower surface region of the exit port, whereby molten steel flowing across the upwardly dish-shaped bottom surface is directed outwardly and upwardly from the nozzle structure, and a submerged entry nozzle (synonymous with “immersion nozzle”) designed such that the exit port has an upper region partially defined by a downwardly slanted lip, whereby a flow of molten steel across the lip is directed outwardly and downwardly into an exit flow of molten steel along the upwardly dish-shaped bottom surface. However, in the Patent Document 3, it is intended to concentrate a molten steel stream in a specific direction, with a view to eliminating retention of argon gas, etc. Thus, it cannot be expected to obtain an effect of uniforming and straightening a molten steel stream flowing out of the discharge port to solve the various problems, such as mold powder entrapment.